The invention relates to a process for registering temperature differences for the purpose of regulating the flatness of a cold rolled metal strip, in particular a thin strip or a foil, in a cold rolling mill by means of an infra-red detector. The infra-red detector is situated a distance from the metal strip and, with respect to the direction of throughput of the strip, after the roll gap formed between two work rolls. The detector has the purpose of determining local temperature differences linearly over the whole width of the strip and converting the infra-red radiation into electric control signals to actuate means for regulating the roll gap. Further, the invention relates to a cold rolling mill with infra-red detector for carrying out the process.
Insufficient flatness of cold rolled metal strip results in kinking and bulging either immediately after rolling or later when the strip is processed further.
A sufficient degree of flatness is therfore one of the most important requirements of cold rolled metal strip, and is given particular attention during quality control. Insufficient flatness can have its origin in the strip entering the roll gap if the entering strip is not completely flat, and/or as a result of incorrect cold rolling. In industry different alloys or the same alloy with different hardness are often rolled to different widths; this, among other things concerning the flatness of the cold rolled strip, can repeatedly lead to problems.
Insufficient flatness of a cold rolled metal strip can be due to further process-related factors:
Too low a pressure setting, the result of which is the formation of "longitudinal waves". PA1 Undersired bending of one or both work rolls, causing the metal strip to be concave or convex in cross-section. PA1 Adjusting devices located at the inlet-side with inaccurately set distance, resulting in strip that is wedge-shaped in cross-section. PA1 Wrongly controlled spray jets for applying cooling medium-leading to metal strip of irregular cross-section. PA1 Working roll/rolls of insufficient surface quality, resulting in irregularities running in the longitudinal and transverse direction in the strip.
The last mentioned problem can be solved only by changing the work roll/rolls and will not be considered herein.
The other factors leading to insufficient flatness can be eliminated by applying means that were previously actuated manually, today fully automatically. These means, for the examples given above, are: increasing the load applied by the work rolls, altering the bending moment, shifting one or both inlet-side devices for adjusting the work rolls, and altering the amount of cooling medium supplied by one or more nozzles. A further means is to employ backing rolls of variable geometry, capable, for example, of adopting different cambers via hydraulic pressure. Of course it is also possible to apply a plurality of means at the same time via an automatic control facility.
The deformation or plastic deformation of the metal during cold rolling produces an increase in temperature which is proportional to the reduction made during the cold rolling pass. The increase in temperature of the cold rolled metal strip is therefore also a measure of the deformation. A non-uniformly deformed i.e. non-planar metal strip is therefore not equally warm at all places. Local temperature differences can be measured, the temperature differences in fact being more important than the actual temperature.
Measured temperature differences in the strip can be converted to electric impulses or signals to actuate the above mentioned means of regulating the roll gap and with that the flatness of the cold rolled strip.
The cold rolled strips emerging from the rolling mill radiate, like all bodies, electromagnetic waves the intensity of which is a function of the surface temperature.
This phenonenom is called thermal radiation, the wave-length of which lies in the infra-red range. Also rising with temperature is the emitted thermal radiation which is specific to the object.
Described in the British patent document GB-PS No. 1 465 792 is a process for regulating the flatness of strip material during cold rolling in a rolling mill, wherein the temperature measurement encompasses the whole width of the moving strip.
The temperature is measured on the strip immediately after the roll gap. An infra-red camera for measuring the thermal radiation converts the measured temperature differences into electric signals to set the roll gap at a constant value, this by employing the inlet-side setting devices and/or the spray nozzles for the coolant as the control means.
This process exhibits the considerable disadvantage that the emissivity of the metal strip emerging from the roll gap is very small in the infra-red range. With reference to the ideal emissivity of a black body the infra-red emissivity of metal strips are only in the range of about 0.05-0.25, depending on the roughness of the surface.
Because this infra-red emissivity is so low, the measurement and control instruments are correspondingly sensitive and suffer relatively large scatter.